Dominating the King's Island, which is the oldest part of Limerick City, is St. Mary's Cathedral, founded in 1168 by the King of Munster, Donal Mor O'Brien, on a site where his palace had stood.

The Cistercian architecture is plain and simple and many purists of ancient church architecture still marvel at it. In the course of the eight centuries which followed its foundation, changes were inevitably effected from time to time and, unashamedly, new parts were added and structures changed. Parts of the original palace are incorporated into the overall design, particularly near the West Door.

The building contains many fine antiquities, all worthy of interest: the pre-Reformation limestone altar (the longest and heaviest of its type in these islands), the Leper's Squint, the Reardos of the High Altar carved by the father of Padraigh Pearse, the 15th century Misericords (mercy seats used by the clergy during services), carved from black bog oak and, it is believed, the only such set in Ireland. The Ffox Memorial and the Budstone Sedilia also date back more than 500 years, and in the Sanctuary can be seen the stone effigies of the Fourth Earl of Thomond and his wife. Also in the Sanctuary is the massive stone coffin lid of the founder of the Cathedral, Donal Mor O'Brien, who died in 1194.